Hi there!
On latest Ubuntu (19.10 and 18.04 also) with installed NM 1.16.0 out of the box I found myself struggling in problem of reapplying WiFi interface changes after changing them.
Here how it looks like:

In my case those commands coming quickly from external application (VPN client) and eventually end up with this error.
But sometimes it just works. Judging by 'seen-bssids' mentioned, problem is probably caused by always changing wireless network environment. Some networks may become out of reach or new network can arise from nonexistence.
Is there any way to deal with such kind of problem without upgrading/downgrading NM?
Should it be fixed in NM?

Yes, it's a bug. When reapplying a connection to a device, we do a diff with the currently active connection and check that the changed properties match a whitelist of properties we are able to change on the fly. The seen-bssids is a synthetic read-only property that lists all the bssids seen for that connection and should be ignored during a reapply. I'll prepare a patch.